The present invention relates to a method for the utilization of carbon tetrachloride. More particularly, the invention relates to a method for the utilization of carbon tetrachloride, which may cause a problem of environmental pollution, in the preparation of methyl chloride as a useful chemical compound.
As is well known, as one of the serious issues in recent years relative to environmental pollution, carbon tetrachloride is, besides specific fluorocarbon compounds and the like chemical compounds, notoriously destructive against the stratospheric ozone layer.
According to an established international agreement, certain fluorocarbons are to be banned in the near future or even within this century, and intensive investigations are now under way in various industrial fields for replacing fluorocarbons with harmless substitutes to comply with this requirement. It should be noted here, however, that several chlorine-containing organic comounds are comparable with or even worse than the fluorocarbon compounds in respect of the destructiveness against the stratospheric ozone layer. Carbon tetrachloride is one of such chlorine-containing compounds. For example, the ozone-destruction index of carbon tetrachloride is estimated to be 1.0 to 1.2 assuming that the values for trichlorofluoromethane and dichlorodifluoromethane are each 1.0. In addition, carbon tetrachloride is suspected to be responsible for the tendency toward global warming by the greenhouse effect for which carbon dioxide is assumed to have the principal responsibility. Accordingly, it is an inevitable trend that carbon tetrachloride is also entirely placed under a ban sooner or later.
As is known, carbon tetrachloride is produced in two industrial processes. One of the processes is the reaction of carbon disulfide with chlorine according to the reaction equation EQU CS.sub.2 +3Cl.sub.2 .fwdarw.CCl.sub.4 +S.sub.2 Cl.sub.2.
This reaction is a single-step reaction producing only carbon tetrachloride and sulfur chloride. Since sulfur chloride can be produced also in a different process, there will hardly be caused any problem in the chemical industry even when the process utilizing the above reaction is entirely discontinued.
Alternatively, carbon tetrachloride is being manufactured by the direct chlorination of methane with chlorine. The reaction here proceeds as a successive chlorination of methane concurrently producing methyl chloride, methylene chloride, chloroform and carbon tetrachloride. The reaction mixture after the reaction is usually a mixture of these four chlorides of methane together with unreacted methane. Each of the intermediate chlorination products of methane, i.e. the chloromethanes other than carbon tetrachloride, is an industrially important compound and is consumed in a large quantity. In other words, carbon tetrachloride is produced as an undesirable by-product in the production of these useful intermediate chlorination products of methane. Accordingly, it is a due expectation that there will be caused a great problem in the chemical industry when the above described process for the direct chlorination of methane is entirely discontinued since no other industrially feasible processes comparable to the direct chlorination of methane are known for the production of the intermediate chlorination products of methane. Accordingly, production of carbon tetrachloride as a by-product in this process is, so to say, a necessary evil.
It is therefore an important technical problem to develop an efficient industrial method for the disposal of carbon tetrachloride without causing any problems of environmental pollution, or rather for the conversion of carbon tetrachloride into useful and harmless compounds. Several proposals and attempts have been made hitherto in this purpose. They are, for example, as follows.
Firstly, carbon tetrachloride can be reduced with hydrogen into lower chlorination products of methane such as chloroform and methylene chloride in the presence of a Raney nickel catalyst. This method, however, has serious problems in respect of the low reaction velocity, insufficient life of the catalyst, formation of a large amount of by-products such as 1,2-dichloroethane and the like, so that this method is still at the stage of laboratory investigations.
Secondly, carbon tetrachloride is subjected to a combustion reaction with a fuel gas such as methane, LPG and the like in the presence of air to be converted into carbon dioxide and hydrogen chloride. This process is of course disadvantageous because, even by setting aside the problem of large consumption of the fuel gas, the combustion temperature is so high as to necessitate use of a highly refractory combustion furnace having high resistance against corrosive hydrogen chloride. Moreover, the products of the process are each a material of low added value capable of being produced at low costs in other large-scale industrial processes.
Apart from the above described direct chlorination of methane, methyl chloride can be industrially produced by the reaction of methyl alcohol and hydrogen chloride. This process is usually performed in the gaseous phase in the presence of a solid catalyst because the velocity of the reaction is low in the liquid phase and hardly achieves sufficient utilization of the starting materials. The above mentioned solid catalyst for the vapor-phase reaction of methyl alcohol and hydrogen chloride usually contains a catalytically active metallic element including the elements of the Group IB, e.g., copper, elements of the Group IIA, e.g., magnesium, calcium and barium, elements of the Group IIB, e.g., zinc, cadmium and mercury, elements of the Group VIB, e.g., chromium and molybdenum, elements of the Group VIIB, e.g., manganese, and elements of the Group VIII, e.g., iron, cobalt and nickel, of the Periodic Table in the form of an oxide or halide supported on a solid carrier such as alumina, pumice, kaolin, zeolite, active carbon and the like.
An idea would be to develop a process in which carbon tetrachloride is utilized in an industrial process for the preparation of methyl chloride, possibly, by the reaction with methyl alcohol according to the reaction equation of: EQU 4CH.sub.3 OH+CCl.sub.4 .fwdarw.4CH.sub.3 Cl+2H.sub.2 O+CO.sub.2.(1)
This reaction is a sequential combination of the two elementary reactions expressed by the reaction equations of: EQU CCl.sub.4 +2H.sub.2 O.fwdarw.CO.sub.2 +4HCl (2) EQU and EQU 4CH.sub.3 OH+4HCl.fwdarw.4CH.sub.3 Cl+4H.sub.2 O. (3)
Namely, carbon tetrachloride is first hydrolyzed in the reaction (2) by reacting with atmospheric moisture and the hydrogen chloride produced by the reaction (2) pertains to the esterification reaction of methyl alcohol according to the equation (3).
In so far as the matter concerns the esterification reaction of methyl alcohol only, no particular difference can be seen in the catalytic performance of the above described various catalysts and satisfactory results can be obtained with any one of those catalysts. In the reaction of methyl alcohol with carbon tetrachloride, in which the esterification reaction must be preceded by the hydrolysis reaction of carbon tetrachloride, however, no quite satisfactory results have yet been reported for the conversion of methyl alcohol into methyl chloride at an industrially feasible velocity in a high yield. This is because the rate-determining reaction in the sequence of the reactions (2) and (3) is the step of the reaction (2) which is of a quite different nature from the reaction (3).
Japanese Patent Kokai 56-167628 and 57-165330 disclose a process for the reaction of methyl alcohol in the vapor phase with hydrogen chloride gas containing methylene chloride, chloroform and carbon tetrachloride in the presence of a zinc chloride catalyst supported on an alumina carrier. This process, however, is for the preparation of methyl chloride from methyl alcohol by the reaction with hydrogen chloride gas, which is a by-product in the direct chlorination of methane and necessarily contains the chloromethanes when no isolation step is undertaken to remove the chloromethanes. In this process, accordingly, it is assumed that the chloromethane compounds, i.e. methylene chloride, chloroform and carbon tetrachloride, are contained intact in the reaction product.
The teaching in the above mentioned Japanese Patent Kokai 56-167628 is given with an object to prevent deactivation of the zinc chloride catalyst on alumina by the deposition of carbon produced by the decomposition of chloroform and carbon tetrachloride while Japanese Patent Kokai 57-165330 teaches a method in which decomposition of methylene chloride, chloroform and carbon tetrachloride can be completely prevented, according to the examples, by using a catalyst of zinc chloride on an alumina carrier.